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equal?

With a perfect imaging system the answer would be 'yes'. However, there are certain factors that prevent this. Firstly, activity in the heart will be attenuated to a greater or lesser extent by soft tissues. For instance in women, photons arising from the anterior myocardial wall will in many SPECT projections have to traverse the breast. A certain number of photons will be absorbed or scattered in the breast and will not be detected by the gamma camera. Because fewer anterior wall counts will be detected, the reconstructed images will contain fewer counts in the anterior wall (i.e., there will be an apparent anterior wall defect). In men, the inferior wall often demonstrates decreased activity because of attenuation from infra-diaphragmatic structures. Secondly scattered photons emanating from outside the heart will be detected as originating from within the heart causing inhomogeneity of uptake. Lastly, resolution in deeper structures is poorer than resolution of more superficial structures contributing to non-uniformity. There are various approaches to correct for this non-uniformity.

Transmission imaging with an external radioactive source can be used to correct for the effects of variable attenuation (Fig. 20). A gadolinium-153 sealed source(s) is mounted on the gamma camera. The camera is rotated about the patient and a transmission map (which can be thought of as a low resolution CT scan) is reconstructed from the projection data. With the transmission data in hand the emission (SPECT) data can be corrected for variable attenuation. This technique is not as yet being used on a widespread clinical basis.

Additional Reading

1. Beller GA, Zaret BL. Contributions of nuclear cardiology to diagnosis and prognosis of patients with coronary artery disease. Circulation 2000; 101:1465-1478. Excellent review of myocardial perfusion imaging.

2. Bergmann SR. Cardiac positron emission tomography. Semin Nucl Med 1998; 28(4):320-340.

A review of cardiac PET applications.

3. Berman DS, Germano G, Shaw LJ. The role of nuclear cardiology in clinical decision making. Semin Nucl Med 1999; 29(4):280-297.

An excellent article focusing on risk stratification.

4. Botvinick EH, ed. Unit 2: Pharmacologic stress and associated topics. In : Nuclear

Figure 19. The relationship of pre- and post-test probability is shown for a test with 90% sensitivity and specificity. For example, if the pretest probability of disease is 0.6 and the test is positive, the post-test probability of disease will be 93% (top curve). If the test is negative, the likelihood of disease is 14% (bottom curve).

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